Glass, transparent to background video
Related answer.

To only gain the reflection and refraction in the sphere, you have to simulate your environment.


Roughly model the shapes of your environment.
Assign a material to the environment object. Use and Emission BSDF. Input the background image as a texture with Window Texture Coordinate.
In the viewport, the image will be mapped to the object according to its position in the (2D) window. From camera view, this will look correct.
Since the camera is not supposed to render the environment object, turn off Camera in the object cycles settings.




From BlenderGuru:


  Just like with the Camera coordinates above, the Window coordinates
  are not used very often. In the past I’ve used this to stick camera
  footage onto a plane behind my scene, so that some glass materials
  would correctly refract what’s behind them and the models would be
  more accurately lit by the environment in the footage, but that’s
  about it.


The render will be more realistic if you add more geometric and have more than just the window frame textured since the reflection of a glass sphere will reflect 360°.